Is there much of difference in mpg between a 4.8L and 5.3L?

I just bought a 2008 Silverado LS with the 4.8L V8. Considering I wanted to add a camper shell, bedliner, rear sliding window, 6" assist steps, and lockable tailgate... I had less money to work with. Originally, I wanted to get the 5.3L, but after adding in the options and tax/license fees, I was pushing my original budget.

My question is, is the long run, would the 5.3L with the Active Fuel Management save me a lot more fuel and money? It's pretty much too late now since I already bought the truck, but I'm just curious.

I don't really plan on using the truck as a commuter. It's mainly for weekend house projects, and a backup vehicle in case my reliable Honda ever does die out on me.